The present invention relates to a hydrodynamic oil-impregnated sintered bearing unit having a higher accuracy of rotation, a higher rotation stability, and an improved durability, and more particularly to a hydrodynamic oil-impregnated sintered bearing unit which is suitable for use as a spindle supporter in spindle motors for information equipment, for example, a motor for driving an optical disc unit such as CD-R, CD-RW, DVD-ROM, or DVD-RAM, a magneto-optical disc unit such as MD and MO, and a magnetic disc unit such as HDD, or a polygon scanner motor or the like of a laser beam printer (LBP).
For the spindle motors of the above stated information equipment, there has been a demand for a higher speed capability, lower costs, lower noise in addition to-higher accuracy of rotation. One of the key elements that determine those high performance requirements is a bearing for supporting a spindle of a motor. Conventionally, as a bearing of this type, a ball bearing or an oil-impregnated sintered bearing have been used.
However, a spindle motor of the type is usually operated at a rotational speed as high as 5000 to 12000 rpm. Particularly, a polygon scanner motor used in an LBP is mostly operated at a rotational speed as high as 20000 to 35000 rpm. Thus, the technology of a ball bearing cannot meet requirements such as noise reduction and cost reduction any longer. Moreover, there has been a strict demand for higher and higher accuracy of rotation, that is, lower shaft vibration, lower NRRO (Non Repeatable Run Out), and lower jitter. It is thus becoming increasingly difficult to meet the above-described high performance requirements with the technology of a oil-impregnated sintered bearing.
Nowadays, from the above point of view, as a bearing of the type, a hydrodynamic oil-impregnated sintered bearing has come to be partly in practical use. This bearing is, for example, composed of a bearing body made of a porous sintered metal impregnated with a lubricating oil or lubricating grease, and non-contactingly supports a spindle by forming a lubricating oil film in a bearing gap by exploiting a dynamic-pressure effect exerted by a dynamic-pressure groove formed in the bearing surface. This construction is capable of coping with the above-described high performance requirements.
However, low-profile information equipment typified by notebook-type personal computers require spindle motors used therewith to be made slimmer and slimmer. Thus, the space volume around a bearing portion tends to be decreased. Moreover, rotational speed is on the increase, which accordingly causes the temperature of the bearing to rise during operation. Thus, the amount of oil exuding from the bearing body due to thermal expansion is increased, and the gap between the sealing washer and the shaft, which is provided for prevention of oil leakage, is inconveniently filled with the seepage of oil. In this state, when the operation comes to a halt, the oil accumulated between the sealing washer and the bearing body is collected in the bearing body again by its thermal contraction, but the oil accumulated between the shaft and the sealing washer might remain uncollected due to capillarity. That is, the inner portion of the bearing unit is sealed with the oil accumulated between the shaft and the sealingwasher. In this state, when the operation is resumed, the airbetween the sealing washer and the bearing body is expanded. Therefore, the oil accumulated between the shaft and the sealing washer is pushed out of the bearing unit and is, as a rotation occurs, scattered around. As a result, the periphery of the bearing is contaminated, and simultaneously a repetition of this phenomenon causes the amount of oil to decrease. This leads to oil shortage and makes it impossible to attain an adequate dynamic-pressure effect.
To cope with such inconveniences, some measures can be considered. For example, applying a commonly-used oil repelling agent, such as a silicone-based oil repelling agent or a PTFE-based oil repelling agent, to the sealing washer or the outer circumferential surface of the shaft opposed thereto, or spraying such an oil repelling agent diluted in an organic solvent to the same. In a case where the shaft is subjected to oil repelling treatment, it is preferable to apply an oil repelling agent to the entire surface of the shaft by immersing the shaft in an agent solution in consideration of the operation efficiency. In this case, however, the processed film is unduly thick and the film thickness varies from several xcexcm to 10 xcexcm. This makes it difficult to control the bearing gap. If the application is conducted by spraying, the film thickness varies more greatly, making the control of the bearing gap more difficult.
Moreover, those oil repelling agents have weaker adhesive strength with respect to a base material and thus exert poor durability. Furthermore, when exposed to oil constantly, the oil repelling agents may possibly be dissolved in oil gradually and consequently the oil repelling effect is deteriorated with the passage of time. This makes it difficult to obtain a stable oil repelling effect for a long period of time.
An object of the present invention is therefore to provide a low-cost hydrodynamic oil-impregnated sintered bearing unit capable of preventing the leakage of lubricating oil to the outside of the unit for a longer period of time with stability, wherein a bearing gap is easily controlled.
To achieve the above object, a hydrodynamic oil-impregnated sintered bearing unit according to the present invention includes: a hydrodynamic oil-impregnated sintered bearing made of a sintered metal and composed of a bearing body having a radial bearing surface opposed via a bearing gap to an outer circumferential surface of a shaft, the bearing body being impregnated with a lubricating oil or lubricating grease, the hydrodynamic oil-impregnated sintered bearing non-contactingly supporting the shaft by exploiting a dynamic pressure effect generated by a relative rotation between the shaft and the bearing body; a housing with its one end opened and its other end closed, the housing having in its inner diameter portion said hydrodynamic oil-impregnated sintered bearing; and a thrust bearing portion for thrust-supporting the shaft, wherein a sealing washer made of a metal or resin material is arranged on an opening-portion side of the housing, and wherein, of a surface of the sealing washer, at least on an inner circumferential surface opposite to the shaft a thin layer of a fluorine-containing polymer is formed.
By forming a thin film of a fluorine-containing polymer at least on the inner circumferential surface of the sealing washer in this way, it is possible to repel oil that is about to exude through the shaft and thus successfully prevent the leakage of oil from the bearing unit.
As the fluorine-containing polymer, for example, a fluoropolyether polymer can be considered that has a main structural unit represented by a formula: xe2x80x94CxF2xxe2x80x94Oxe2x80x94 (x is an integer of from 1 to 4). The average molecular weight of the fluoropolyether polymer preferably falls in the range of 500 to 50000. A fluorine-containing polymer of this type can be formed into a film of even thickness which is far thinner than a bearing gap by simple means, such as application or immersion. This makes the control of a bearing gap easy. Moreover, the fluorine-containing polymer has high adhesion with respect to metal and is thus excellent in durability, and has satisfactory oil resistance and thus offers a stable oil-repelling effect for a long period of time.
In a case where the average molecular weight is no greater than 500, the following inconveniences arise.
(1) The polymer is highly volatile due to an unduly small molecular weight. For example, if there remain unreacted compounds, optical equipment might be contaminated by the volatilization.
(2) As the oil repelling effect is gradually decreased, the preservation stability is deteriorated. Specifically, the smaller the molecular weight, the shorter the molecular length. The molecular is so structured as to have functional groups on both sides of its portion exerting oil repellency. Even if the total length of the molecular is made short, the length of the functional group is kept unchanged. Consequently, the length of the portion exerting oil repellency becomes short and thus the oil repelling effect is decreased. Moreover, since the functional groups have the property of bounding together at high humidities, the greater the concentrations of the functional groups, the easier they bound together. This leads to degradation in the preservation stability.
On the other hand, in a case where the average molecular weight is greater than 50000, the following inconveniences arise.
(1) The entire length of the molecular becomes long, but, as described above, the lengths of the functional groups are kept unchanged. Therefore, the adsorption (reaction) property becomes weak.
(2) An unduly large molecular weight entails high viscosity. This may lead to an increase in the film thickness and to uneven thickness, and thus makes highly precise dimension control impossible.
Similar advantageous effects can be attained in a case where the above stated fluorine-containing polymer thin film is formed on the region of the surface of the shaft opposite to the sealing washer. The above stated fluorine-containing polymer can be formed into a significantly thin film of even thickness. Therefore, even if it is applied to the entire surface of the shaft, the application has little effect on the bearing gap. This makes the control of the bearing gap easy. Moreover, it is possible to improve the efficiency to the application process by employing a full-coating method. In this case, the sealing washer is preferably made of a material in which a contact angle between the material and a lubricating oil to be used is 20xc2x0 or above, or may be subjected to surface treatment to obtain the same effect. This enhances the oil-repelling effect and thus a more satisfactory oil leakage prevention effect can be attained.
Moreover, a satisfactory oil leakage prevention effect can also be attained by forming a thin layer of a fluorine-containing polymer, of the surface of the sealing washer, at least on the inner circumferential surface opposite to the shaft, and, of the surface of the shaft, at least on the region including the portion opposite to the sealing washer.
Furthermore, the same effect can also be attained by forming a thin layer of a fluorine-containing polymer, of the surface of the sealing washer, at least on a top surface, and, of the surface of the shaft, at least on an upper part of a region opposite to an inner surface of the sealing washer.
In either case, by setting the radial gap between the shaft and the sealing washer at 0.1 mm or below, sufficient capillarity can be obtained. This is further advantageous in prevention of oil leakage.
Forming a dynamic-pressure groove inclined with respect to an axial direction on the radial bearing surface makes it possible to form a highly-rigid, stable oil film in the bearing gap. This enhances rotational accuracy.
By forming a ventilating path opening into both end portions of the bearing body between the outer circumferential surface of the bearing body and the inner circumferential surface of the housing, when the shaft is inserted into the inner circumferential hole of the bearing body, the air trapped within the housing is allowed to pass through the ventilating path so as to be discharged from the housing. This helps prevent air from being dragged into the bearing gap and make the insertion of the shaft smooth.
The hydrodynamic oil-impregnated sintered bearing unit described above is suitable for use in an optical disc drive spindle motor for rotating an optical disc by exploiting a relative rotation between the shaft and the bearing body, a magnetic disc drive spindle motor for rotating a magnetic disc by exploiting a relative rotation between the shaft and the bearing body, a polygon scanner motor for rotating a polygon mirror by exploiting a relative rotation between the shaft and the bearing body, or the like. Note that the xe2x80x9coptical discxe2x80x9d mentioned here also refers to a magneto-optical disc, such as MD or MO.
To overcome the above stated technical subject, the present invention is a hydrodynamic oil-impregnated sintered bearing unit including: a rotary shaft body having in a bottom-end portion of its shaft portion a flange portion; a housing portion formed into a sleeve with a bottom having at its one end an opening portion, the housing portion having a thrust bearing gap interposed between its bottom surface and a bottom-end surface of the flange portion; a bearing member having a radial bearing gap interposed in an outer diameter surface of said shaft portion and having a thrust bearing gap interposed between an end surface of the flange portion and an opposite end surface, the bearing member being housed in said housing portion; and a sealing member arranged on an opening-portion side of said housing portion and having a through portion into which said shaft portion is inserted, wherein said bearing member is composed of a porous material of sintered metal in which a lubricating oil or lubricating grease is impregnated, and said bearing member is constructed such that a dynamic pressure is generated in said thrust bearing gap and said radial bearing gap, the bearing unit characterized in that, of a surface of the sealing member, at least on a top surface a coating film of a fluorine-containing polymer is formed.
According to this construction, lubricating oil, after passing through the radial bearing gap and the outer circumferential surface of the shaft portion, reaches the through portion of the sealing member, is then subjected to the capillarity occurring in the through portion, and is repelled on the top surface of the sealing member having a coating film of a fluorine-containing polymer formed. This prevents the leakage of oil to the outside of the bearing unit successfully, and, at the time when the relative rotation between the rotary shaft body and the bearing member comes to an end, the lubricating oil existing in the gap between the inner diameter surface of the sealing member and the outer diameter surface of the shaft portion is surely collected in the bearing member.
As the fluorine-containing polymer, a polymer which is suitable for forming a coating film on the surface of metal or resin, for example, a polyfluoroalkyl polymer or a fluoropolyether polymer is preferable. Moreover, it is preferable that those polymers include a functional group having a high affinity for metal, for example, an epoxy group, an amino group, a carboxyl group, a hydroxyl group, a mercapto group, an isocyanate group, a sulfone group, an ester group, or the like.
Such a fluorine-containing polymer can be formed into a thin film of even thickness with a technique commonly used for application, such as spraying or immersion. Moreover, the fluorine-containing polymer offers high adhesion with respect to metal and is not dissolved in oil easily, and accordingly exerts a satisfactory oil-repelling effect for a long period of time and an improved durability.
Moreover, a similar advantageous effect can be attained in a case where the above stated fluorine-containing polymer coating film is formed, of the surface of the shaft portion, at least on the upper part of the outer diameter surface region opposite to the through portion. Particularly, this fluorine-containing polymer can be formed into a thin coating film of even thickness. Therefore, even if it is applied to the entire or substantially entire area of the surface of the shaft portion in view of simplification of the operation, the radial bearing gap can be controlled with ease.
Further, a similar advantageous effect can be attained also in a case where the above stated fluorine-containing polymer coating films are formed, of the surface of the sealing member, at least on the top surface, and, of the surface of the shaft portion, at least on the upper part of the outer diameter surface region opposite to the through portion, respectively. With an eye particularly to the oil-repelling effect, a synergistic effect of oil-repelling treatment on the sealing member and oil-repelling treatment on the shaft portion significantly enhances the oil-repelling effect. This further improves the oil leakage prevention effect.
In this case, it is preferable that the sealing member be made of a metal material or a resin material, that a gap between the inner diameter surface of the sealing member and the outer diameter surface of the shaft portion be set at 0.1 mm or below, and that the sealing member be made of a material in which a contact angle between the material and a lubricating oil to be used is 20xc2x0 or above, or be subjected to surface treatment to achieve the same effect. This construction has the advantages of a satisfactory oil-repelling effect and optimal oil leakage prevention effect.
Forming a dynamic-pressure groove inclined with respect to an axial direction on the radial bearing surface makes it possible to form a highly-rigid, stable oil film in the radial bearing gap. This provides highly accurate rotational characteristics. Moreover, forming a dynamic-pressure groove inclined with respect to a radial direction on the thrust bearing surface makes it possible to form a stable oil film in the thrust bearing gap. This provides satisfactory thrust bearing performance.
Then, the above stated dynamic-pressure bearing unit is incorporated into a disc-driving spindle motor of information equipment. Consequently, there is realized a spindle motor suitable for use in a disc driving unit that satisfies high performance requirements including higher accuracy of rotation, a higher speed capability, lower costs, and lower noise, for example, an optical disc drive or a magnetic disc drive. Note that the xe2x80x9coptical discxe2x80x9d mentioned here also refers to a magneto-optical disc, such as an MD, MO, or the like.